This invention relates to the detection and measurement of flow characteristics of liquid flowing in a sewer pipe at a location remote from a flow detector.
The occurrence of a sanitary sewer overflow (SSO) during a rain storm has long plagued sewer systems. Such SSOs can occur through manholes or other structures connected to the sewer system. At present SSOs are largely unmeasured and unreported but the Environmental Protection Agency is developing programs which will require reporting and controlling SSOs. Traditionally, the approach to quantifying such overflows is to install two measuring devices, one above and the other below a potential site so that the volume of an overflow can be determined by subtraction. A preferred approach is to provide a single measuring device in a sewer overflow pipe but many overflows occur from manholes as gushers or in locations where the installation of an overflow monitor is not feasible.
During the last century, hydraulic engineers have developed equations called pipe curves which describe the relationship between the depth of an open channel and the velocity of gravity flow. For a given depth of flow, there is a unique and predictable flow velocity (and flow rate) of liquid in the channel which can be determined from a standard pipe curve such as one described by Robert Manning in 1890. Another standard pipe curve is called the Colebrook-White pipe curve.
Under normal conditions, a plot of a collection of paired depth and velocity readings taken periodically over a time interval during which the depth and velocity vary, called a "scattergraph", should produce a pattern similar to that of a standard pipe curve. Flow patterns that deviate from a standard pipe curve tend to indicate either that the flow meter is malfunctioning or that the hydraulics of the pipe are not normal. Heretofore, it has not been possible to analyze automatically a scattergraph of sewer flow data which deviates from the standard pipe curve under abnormal flow conditions to establish the existence and possible cause of the abnormality. While scattergraph analysis has been attempted previously, visual examination of each scattergraph has been required. One published scattergraph analysis technique is "A Guide to Short Term Flow Surveys of Sewer Systems", published by the Water Research Centre, in Wiltshire, England. According to that publication, information regarding abnormal flow conditions is obtained by observation of the spread and shape of the data points in a scattergraph.